Last weekend's heat wave throughout the central US and East Coast forced many runners to wake up before the crack of dawn or take their runs indoors to the treads. With the heat index - what the temperature feels like when you combine air temperature with relative humidity - rising above 100F, several large races such as the NYC Triathlon and NYRR Marathon Training Series 10-miler were even cancelled. With two months of summer left, we will likely experience a few more heat waves, and you can prepare yourself better for the next one.
A heat index above 103F (e.g. 88F & 75% humidity or 94F & 50% humidity) is considered dangerous. Heat cramps and heat exhaustion are likely, and heat strokes are possible with prolonged running. When the index is above 125F (e.g. 92F & 85% humidity), a heat stroke is highly likely as the process of evaporating sweat from your skin is limited, and your body cannot regulate its temperature. This can cause your central nervous system to shut down and lead to brain damage. You might be surprised to know that the heat index is calibrated to only shady conditions, so if you are exposed to direct sunlight the value can be 10-15F higher! Here is a handy Heat Index calculator and chart.
When to Run in the Heat
Since the heat index value is often understated, I recommend athletes to not run when the index indicates "danger" or "extreme danger". Be smart and run on the tread or swap a run day with an indoor cross-training day. Look at the forecast the day before a run and optimize when to run for the lowest heat index, which means sacrificing your beauty sleep and waking up early when the air temperature is low or staying patient until the evening when the relative humidity is low.
There is no benefit complaining about weather that you can't control. What you can control is when you run and your attitude. I spend time with my athletes helping them get into a more relaxed and positive state of mind before hard workouts and races, because I truly believe mental strength breeds physical strength. Other runners' complaints about the weather should not affect you. Run the higher road. In the words of Andre Agassi, "control what you can control".
How to Run in the Heat
Surviving hot conditions will require you to adapt your 1) gear, 2) nutrition, 3) pacing, and 4) mindset. By doing so, you will be fine running outside even when it is hot and the heat index indicates "caution" or "extreme caution".
Keep your clothing to a minimum, e.g. a singlet or sports bra and split shorts. My fellow coaches Tim Downey and Kyle Axman keep it simple by running shirtless at all times. Your kit should be light in color, lightweight, and ideally have vents or mesh. You'll notice many pros cut holes in their singlets for races in the heat. Use nip guards and a healthy amount of lube in areas where you experience chafing. Just be careful where and when you ask for lube - a running specialty store or race expo are fine but Whole Foods or a bar may not be!
In addition, use sunscreen if you anticipate direct exposure to sunlight. A visorwill help keep your head cool and prevent your body from overheating. A light cap is ok; however, a cap without ventilation on the top will be counterproductive by trapping heat on your head. I have forced myself to become accustomed to wearing sunglasses to protect against UV damage and avoid using extra energy from squinting and tensing my face. One of Meb's secrets is that wearing sunglasses also helps reinforce good form - your head is steady and your shoulders back and relaxed to keep your sunglasses secure.
Electrolyte intake is crucial given we lose more electrolytes, and at a faster rate, in the heat. Before, during, and after runs, try having an electrolyte sports drink instead of just water. NuuN tablets and SaltStick are worth first experimenting with and then using routinely. Ample hydration and electrolytes will prevent the common occurrence of "cardiac drift" in the heat, where your heart rate increases over the course of a run even when your effort is the same. Dehydration causes your heart to work harder to pump your blood and deliver oxygen to muscles.
In the heat, I either pre-plan water stops or carry a bottle with an electrolyte drink. While carrying a bottle adds extra weight, I alternate between my right and left hands and use the bottle to reinforce good form, like the sunglasses trick, holding it by my sides instead of by my chest and driving straight up and down with my arms.
With a cautionary heat index, focus on your rate of perceived effort rather than a specific pace. During your first week of running in warmer weather, listen to your body and adapt gradually over the ensuing couple of weeks. It's ok, and arguably more beneficial, if you run a slower pace to maintain the same effort as you did in cooler weather. When heat spikes even post-acclimatization, perceived effort is be your guiding principle. This could include a longer warm-up, cool-down, and recovery breaks or a more intentional run/walk strategyto better manage your heart rate.
You will most likely not be racing a marathon in the heat and humidity, though even if you do, you'll be mentally and physically prepared. I have raced a handful of marathons when it was over 80F at the start, including my first Boston in 2012. Safely use hot conditions to build your mental strength. Your body will experience physiological changes to sweat faster and pump blood more easily, becoming more efficient at cooling itself. The improved blood circulation delivers more oxygen to your muscles, building your mental confidence as well as your aerobic capacity and physical strength.
If you experience symptoms of notable dizziness, lack of sweat, muscle cramps, or nausea, stop your run and cool yourself by seeking shelter and cold water or ice. If symptoms persist then seek medical help. It is not smart to run in peak heat conditions simply to act tough or impress someone. Remember to control what you can control: prepare your gear, nutrition, pacing, and mindset ahead of time, and you will wish it were hotter and more humid out...said no runner ever!